fix: only abort effect flushing if it causes an existing effect to be scheduled

Author: Rich-Harris

.changeset/clever-months-clap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: only abort effect flushing if it causes an existing effect to be scheduled

packages/svelte/src/internal/client/reactivity/batch.js

@@ -28,7 +28,7 @@ import * as e from '../errors.js';
 import { flush_tasks } from '../dom/task.js';
 import { DEV } from 'esm-env';
 import { invoke_error_boundary } from '../error-handling.js';
-import { old_values } from './sources.js';
+import { old_values, schedule_version } from './sources.js';
 import { unlink_effect } from './effects.js';
 import { unset_context } from './async.js';
 
@@ -598,7 +598,7 @@ function flush_queued_effects(effects) {
 		var effect = effects[i++];
 
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
-			var n = current_batch ? current_batch.current.size : 0;
+			var sv = schedule_version;
 
 			update_effect(effect);
 
@@ -619,13 +619,9 @@ function flush_queued_effects(effects) {
 				}
 			}
 
-			// if state is written in a user effect, abort and re-schedule, lest we run
-			// effects that should be removed as a result of the state change
-			if (
-				current_batch !== null &&
-				current_batch.current.size > n &&
-				(effect.f & USER_EFFECT) !== 0
-			) {
+			// if an effect is invalidated by a user effect, abort and re-schedule, lest we
+			// run effects that should be removed as a result of the state change
+			if (schedule_version > sv && (effect.f & USER_EFFECT) !== 0) {
 				break;
 			}
 		}

packages/svelte/src/internal/client/reactivity/sources.js

@@ -299,6 +299,11 @@ export function increment(source) {
 	set(source, source.v + 1);
 }
 
+/**
+ * We increment this value when an effect is scheduled as a result of a state change
+ */
+export let schedule_version = 0;
+
 /**
  * @param {Value} signal
  * @param {number} status should be DIRTY or MAYBE_DIRTY
@@ -334,6 +339,7 @@ function mark_reactions(signal, status) {
 		if ((flags & DERIVED) !== 0) {
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
 		} else if (not_dirty) {
+			schedule_version += 1;
 			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}